1. Field of the Invention
The present invention relates to a method of producing an electrode for a resonator in thin-film technology, in particular to a method of producing a resonator which includes a piezoelectric layer arranged at least partly between a lower electrode and an upper electrode, and here relates, in particular, to the production of a BAW resonator (BAW=bulk acoustic wave).
2. Description of Prior Art
In the production of frequency filters in thin-film technology using thin-layer bulk acoustic resonators (FBAR=film bulk acoustic resonator), which are also referred to as BAW resonators, the piezoelectric layer, e.g. an AlN layer, a ZnO layer, or PZT layer, is typically deposited by means of a reactive sputtering process. The reactive sputtering process is preferred because it requires a relatively low process temperature and offers deposition conditions which are easy to control and reproduce. In addition, the reactive sputtering process leads to a high-quality thin layer.
A problem associated with producing the thin layers arises due to the specific growth conditions of piezoelectric layers, in which crystallites having a certain preferred orientation grow faster than those with other orientations. In combination with the poor edge coverage of a sputtering process, these specific growth conditions of the piezoelectric layers lead to the formation of growth defects at the topology steps.
These growth defects will be explained below in more detail with reference to FIG. 1. FIG. 1A shows an arrangement which includes a substrate 100 comprising a first, lower surface 102 as well as a second, upper surface 104. A first, lower electrode 106 is formed in a portion on the upper surface 104, which electrode 106 in turn includes a first, lower surface 108 as well as a second, upper surface 110. By means of the above-mentioned sputtering process, a piezoelectric layer 112, which is an AlN layer in the example depicted, has been produced on that portion of the upper surface 104 of substrate 100 which is not covered by the electrode 106 as well as on the upper surface 110 of electrode 106.
As may be seen from FIG. 1A, due to the arrangement of electrode 106 on surface 104 of substrate 100, a step 114 (topology step) is formed whereat a growth defect occurs during a sputtering process due to the poor edge coverage of the sputtering process and due to the specific growth conditions of the piezoelectric layer 112, which growth defect is generally indicated by reference numeral 116 in FIG. 1A. By reference numeral 118, FIG. 1A depicts a preferred direction of growth of the piezoelectric layer in the various areas. In the area of step 114, these lines 118 may be seen to be offset, which has led to the occurrence of the growth defect 116. The lines of offset and the defects resulting therefrom are undesirable for the reasons stated below and lead to problems regarding the reliability of the device to be produced, in particular in connection with the subsequent deposition of an upper electrode.
Specifically, in a subsequent deposition and structuring of a metallization for producing the upper electrode, a metallic spacer will remain which may subsequently lead to electrical short-circuits, whereby the functionality of the device, e.g. a filter, may be degraded or completely destroyed. FIG. 1B represents the structure which results, starting from FIG. 1A, once a metallization which has been subjected to full-area deposition, has been structured for producing an upper electrode. FIG. 1B shows a second, upper electrode 120 formed on a surface 122 of the piezoelectric layer 112 which is facing away from substrate 100, such that this upper electrode 120 is at least partly opposite the electrode 106. The thin-layer bulk acoustic resonator is formed in that area in which electrode 106 and upper electrode 120 overlap. As may also be seen in FIG. 1B, a rest of metal 124 (metal spacer) has remained in the area of the growth defect 116. This metal spacer 124 leads to the above-mentioned problems in connection with electric short-circuits and the like.
A further disadvantage of the structure described in FIG. 1 is that for suppressing undesired spurious modes, a concept is usually applied wherein an area outside the upper electrode 120 has, across a width of several micrometers, a defined geometry, i.e. a defined thickness. As may be seen in FIGS. 1A and 1B, this may only be achieved if the upper electrode 120 is configured to be clearly smaller than electrode 106, so that hereby an increase in the size of the structure is required for suppressing the undesired spurious modes, which increases the overall size of the structure. In addition, this significantly degrades the behavior of the resulting resonator device with regard to bandwidth and parasitic capacitances.
Another problem arising in connection with the production method described with regard to FIG. 1 is that a weak point of the piezoelectric layer exists in those areas in which the upper electrode 120 traverses the growth defects, which is always necessary, the weak point being related to a dielectric breakdown. Due to the growth defect 116 (void-defect) the desired ESD resistance (ESD=electrostatic discharge) which would be expected in a fully planar arrangement therefore cannot be achieved.
The method described with regard to FIG. 1A and, in particular, the problems in the prior art associated therewith have, in principle, not been discussed so far. In principle, the metal spacer 124 may be reduced by a pronounced over-etching in structuring the upper electrode 120. Due to the significant overhang of the piezoelectric layer 112, the metal spacer can still not be fully removed despite etching. The removal can only be achieved by subjecting the upper electrode to further isotropic etching, e.g. wet-etching. The disadvantage of this approach, however, is that due to an underetching of the resist mask, which is hard to control, the resulting electrode edge is poorly adjusted in relation to other layers, which, again, leads to a deterioration of the spurious mode behavior.
It shall be pointed out here that substrate 100, in turn, may typically consist of a sequence of several layers. These layers, for their part, may be structured and thus contribute themselves to the creation of topology steps. A typical example is a so called acoustic mirror, i.e. a sequence of layers having high and low acoustic impedances. For simplifying the representation in FIG. 1, these details have been dispensed with, and substrate 100 has been drawn as a homogeneous block.